Most conventional coating resins are insoluble in water. Therefore, in general practice, they have been dissolved in a suitable organic solvent or dispersed in water with the aid of an emulsifying agent or surfactant in order to provide a coating composition suitable for application to a substrate surface. A serious disadvantage of organic solvent solutions is that they are potentially toxic, flammable and environmental pollutants.
Water-reducible coatings are highly desirable because they do not present these problems. For this reason, various water-reducible coating resins, such as the one described in U.S. Pat. No. 4,474,926, have been developed. Water-reducible coatings which utilize such resins have been developed for a variety of purposes and have been widely accepted in many applications such as highway striping paint.
U.S. Pat. No. 4,968,741 describes a coating for metal substrates which provides improved corrosion and rust resistance. Such coatings are of the water-reducible type and can be beneficially utilized in the automotive industry and other applications where good rust resistance is needed. For instance, such coatings are excellent for coating bridges and other outdoor metal structures.
There are many applications where it is highly desirable for a water-reducible coating to offer improved flexibility and ultraviolet light resistance in addition to good corrosion and rust resistance. For example, such water-reducible coatings are valuable in roof coating applications. U.S. Pat. No. 5,194,469 relates to such a water-reducible coating which offers improved flexibility and ultraviolet light resistance in addition to outstanding corrosion and rust resistance.
It is important for coatings applied to concrete structures to be strong and elastomeric. Good durability is required in order for the coating to have a long service life. Since concrete is highly prone to cracking, it is highly desirable for the coating to have the ability to bridge cracks that form in the concrete structure. In the case of metal (steel) reinforced concrete, it is also highly desirable for the coating to prevent moisture from reaching the metal reinforcing rods. However, this combination of properties has proven to be elusive.
For purposes of this patent application, an aqueous coating system is considered to be a colloidal dispersion of a resin in water which can be reduced by the addition of water and which forms a durable coating when applied to a substrate surface. The term aqueous coating system is used herein interchangeably with the term water-reducible coating. Other names which are sometimes applied to water-reducible coatings are waterborne, water-solubilized and water-dilutable.